It is generally necessary to suppress to a suitable value the inductance of a large superconductive magnet used for nuclear fusion reactors, MHD generators, and superconductive magnetic energy storage. Consequently, the rated current must necessarily be a high current of over 10 kA. With such a large superconductive magnet, high reliability is required from the perspective of safety, and the superconductor used for this must be designed to be fully stable. That is, the conductor must be so designed that it is capable of returning to the super conductive state after a cause has been eliminated, even when such cause broke the superconductive state of the superconductor leading to the transition to the resistive state. For this purpose, a great amount of a stabilizing metal is formed composite with the superconductor. Such a large superconductive element must further have sufficient strength to withstand the large electromagnetic force exerted on the coil. It thus becomes necessary to make the super conductor itself composite with a reinforcing material. When a large superconductor is designed based simply on the principle of small or medium sized superconductors which have been available heretofore, the conductor has an extremely large conductive area and a low current density, fails to provide suitable applicability to nuclear fusion reactors, MHD generators, and so on due to large size of the superconductive magnet incorporating this, and becomes uneconomical and impractical.
Various other types of superconductors have thus been proposed, all of which permit a high current density and have high stability and high resistance to stress. As an example, it has been proposed to use aluminum as a stabilizing metal which has a small magnetic reluctance in a high magnetic field. However, aluminum is low in mechanical strength and has high peizoelectric resistance effects, so it becomes necessary to connect an aluminum layer with a large amount of a reinforcing material so that the resultant composite structure has high resistance to stress. The use of aluminum as a stabilizing metal does not contribute to a higher current density at all. As an another method, it has been proposed to form a number of projections on the surface of the stabilizing metal of a composite superconductor, for example, to form parallel grooves (longitudinal grooves) along the longitudinal direction of the conductor and to form a group of transverse grooves crossing these longitudinal grooves, so as to increase the cooling area contacting the cooling medium and to improve the cooling efficiency of the surface of the stabilizing metal, thereby reducing the amount of the stabilizing metal used and increasing the current density. The cooling characteristics improve when such grooves are formed in the stabilizing metal to increase the cooling area. However, the cooling characteristics do not improve in proportion to the increase in the cooling area, but show saturation. Formation of such a number of grooves by decreasing the distance between the grooves to increase the cooling area requires special processing equipment. The drop in processing efficiency leads to an increase in cost. The great electromagnetic force is exerted under the superconductive condition as a surface pressure on the conductor surface through a spacer arranged for insulation. This results in deformation and crushing of the projections, that is a kind of fins, between the grooves when they are thin so that the cooling effects are degraded.